1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a sheet and an image forming apparatus having such a sheet conveying apparatus.
2. Related Background Art
In the past, many sheet conveying apparatus have been incorporated, for example, in a main body of an image forming apparatus.
As shown in FIG. 10, such a sheet conveying apparatus is designed so that a (one-sided copying) sheet having one surface on which an image is formed by image forming means of an image forming apparatus (not shown) is discharged outside as it is or is reversed and then is discharged or is reversed and then is sent to the image forming means again in order to form an image on the other surface of the sheet (two-sided copying).
Now, an operation of a conventional sheet conveying apparatus 100 will be explained.
(Face-up sheet discharging mode in one-sided copying)
In the image forming apparatus, a sheet to which a toner image has been fixed is sent to an outer sheet discharge roller pair 27 by an inner sheet discharge roller pair 25 and then is discharged, by the outer sheet discharge roller pair 27, onto a sheet discharge tray (not shown) of a main body 101 of the image forming apparatus with its image bearing surface facing upwardly. In this case, a flapper 26 disposed at a branched portion is switched to a position shown by the solid line.
(Face-down sheet discharging mode in one-sided copying)
By switching the flapper 26 to a position shown by the broken line, the sheet sent from the inner sheet discharge roller pair 25 is conveyed toward a conveying path 66 and then is sent in a direction indicated by the arrow A by a sheet discharge reverse roller pair 59. At a time when a trailing end of the sheet reaches in a range La, the sheet discharge reverse roller pair 59 reversely rotates to send the sheet to the outer sheet discharge roller pair 27. Then, the sheet is discharged, by the outer sheet discharge roller pair 27, onto the sheet discharge tray (not shown) of the main body 101 of the image forming apparatus with its image bearing surface facing downwardly.
(Two-sided copying mode)
Similar to the face-down sheet discharging mode, by switching the flapper 26 to the position shown by the broken line, the sheet sent from the inner sheet discharge roller pair 25 is conveyed toward the conveying path 66 and then is sent in the direction indicated by the arrow A by the sheet discharge reverse roller pair 59 and a two-sided reverse roller pair 60. At a time when the trailing end of the sheet reaches in a range Lc, the two-sided reverse roller pair 60 reversely rotates to sent the sheet to a sheet re-feeding path 50 with its image bearing surface facing upwardly.
By the way, in the two-sided copying mode, a sheet conveying sheet in such a sheet conveying apparatus is selected to be higher than a sheet conveying speed in the image forming apparatus in order to increase the image forming speed.
Now, a method of controlling the sheet conveying speed in this case will be briefly explained.
An inner sheet discharge sensor 62 is disposed downstream of the inner sheet discharge roller pair 25. Fixing means 24 of the image forming apparatus and the inner sheet discharge roller pair 25 are rotated by a common motor M3. The sheet discharge reverse roller pair 59 also having a surface reverse function is rotated by a forward and reverse rotatable and speed-controllable pulse motor M4. Further, the two-sided reverse roller pair 60 also having a surface reverse function is rotated by a forward and reverse rotatable and speed-controllable pulse motor M5.
When a leading end of the sheet is detected by the inner sheet discharge sensor 62, control means (not shown) calculates a time when the trailing end of the sheet surely leaves the inner sheet discharge roller pair 25. At that time, motors M4, M5 are brought to double speed. Thereafter, the two-sided reverse roller pair 60 reversely rotates so that the double speed control is continued until immediately before the sheet abuts against a second registration roller pair (not shown) disposed upstream of the image forming means of the image forming apparatus.
However, in the conventional sheet conveying apparatus 100, the leading end of the sheet is detected by the inner sheet discharge sensor and the control means calculates the time taken till the trailing end of the sheet leaves, thereby determining the speed control and the stop point in the reverse rotation. Therefore, slip in the roller pairs due to change in a sheet surface characteristics caused by change in an environmental condition and sheet delay due to wear of the roller pairs must be taken into consideration.
Thus, the switching to the double speed must be set with some play, thereby the total processing speed in the two-sided copying mode is reduced.
Further, also regarding the reverse position, since the same factors must be taken into consideration, dispersion in the reverse position must be made allowance for. Thus, the excessive length including the dispersion amount and a (dimensions La, Lc in FIG. 10) must be reserved for the reverse position, thereby making the sheet conveying apparatus and the image forming apparatus having such a sheet conveying apparatus bulky.
An object of the present invention is to provide a sheet conveying apparatus in which a length required for surface reverse is minimized by optimizing a timing for conveying a sheet at a high speed and by increasing a processing speed in a two-sided copying mode and by reducing dispersion in a reverse position, thereby making the sheet conveying apparatus compact without increasing a manufacturing cost, and an image forming apparatus having such a sheet conveying apparatus.
To achieve the above object, a sheet conveying apparatus according to the present invention comprises sheet constant speed conveying means for conveying a sheet at a predetermined speed, a sheet conveying path for guiding the sheet being conveyed by the sheet constant speed conveying means, a sheet reverse feeding rotary member pair for forwardly rotating to convey the sheet conveyed on the sheet conveying path downstream and then reversely rotating to feed the sheet reversely, a surface reverse conveying path branched from the sheet conveying path and for guiding the sheet reversely fed by the sheet reverse feeding rotary member pair, trailing end detecting means disposed between the sheet constant speed conveying means and the sheet reverse feeding rotary member pair and for detecting a trailing end of the sheet being conveyed on the sheet conveying path, and control means for controlling rotation of the sheet reverse feeding rotary member pair in such a manner that the sheet can be conveyed at a higher speed than the predetermined speed when the trailing end of the sheet is detected by the trailing end detecting means.
In the sheet conveying apparatus, the sheet is conveyed at the predetermined speed on the sheet conveying path by the sheet constant speed conveying means. The sheet is then conveyed by the sheet reverse feeding rotary member pair.
The control means serve to rotate the sheet reverse feeding rotary member pair at a high speed to convey the sheet at the higher speed than the predetermined speed when the trailing end of the sheet being conveyed on the sheet conveying path is detected by the trailing end detecting means. In the sheet conveying apparatus according to the present invention, the control means can store a time period from when the trailing end of the sheet is detected by the trailing end detecting means to when the sheet passes by a branched portion between the sheet conveying path and the surface reverse conveying path. The control means controls the reverse rotation of the sheet reverse feeding rotary member pair on the basis of the time period.
The image forming apparatus according to the present invention comprises the aforementioned sheet conveying apparatus, and image forming means for forming an image on the sheet conveyed by the sheet conveying apparatus.
In the image forming apparatus according to the present invention, the surface reverse conveying path may be a sheet re-feeding path having a re-feeding roller pair, and a downstream end of the sheet re-feeding path may be connected to a sheet feeding path to the image forming means.
The image forming apparatus may be designed so that, after the image is formed on one surface of the sheet by the image forming means, the sheet is conveyed at the high speed on the surface reverse conveying path and surface-reversed, and then, the sheet is conveyed on the re-feeding path by the re-feeding roller pair to be guided into the sheet feeding path, and an image is formed on the other surface of the sheet by the image forming means.
In the image forming apparatus according to the present invention, the re-feeding roller pair may be rotated at a high speed by the control means.
The image forming apparatus may be designed so that, after the image is formed on one surface of the sheet by the image forming means, the sheet is conveyed at the high speed on the surface reverse conveying path and surface-reversed, and then, the sheet is conveyed at a high speed on the re-feeding path by the re-feeding roller pair to be guided into the sheet feeding path, and an image is formed on the other surface of the sheet by the image forming means.